Heartworm infection in dogs

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Heartworm (Dirofilaria immitis) infection in dogs has been reported in all U.S. states. The highest numbers are in the Southeast and Mississippi River Valley with infection rates in some areas as high as 45 percent. In 2001, approximately 240,000 cases were diagnosed in the U.S. The mosquito is an essential part of the heartworm life cycle. This means that the incidence of heartworm varies between regions and climates relative to the mosquito populations. In Colorado, the regions of most concern are Grand Junction and Front Range areas. Dogs living in or traveling to these areas, especially during the mosquito season are at higher risk. Fortunately there is a simple blood test to detect heartworm infection and there are effective preventive drugs.

The life cycle of the heartworm is interesting and complex. In a dog infected with heartworms, adult heartworms live in the pulmonary (lung) arteries and to a lesser extent in the right ventricle of the heart. After mating, the adult heartworm releases microfilaria (baby heartworms) into the blood. Female mosquitoes ingest these microfilaria while feeding. Then the microfilaria undergo two molts over 8-17 days in the mosquito. At this stage, they can be injected back into a dog at another feeding where a third molt occurs over 1-12 days. A final molt occurs over 50-68 days to become an immature adult. Immature adults enter the bloodstream and migrate to the heart and lungs where they mature. Amazingly, male heartworms grow to about 15-18 cm and the female to 25-30 cm. Mating occurs and production of microfilaria begins. The complete life cycle takes about 184-210 days with microfilaria found in the circulating blood at 7-9 months. Adult heartworms are known to live 5-7 years and microfilaria up to 30 months. Interestingly, heartworm molting and maturation are dependent on the presence of a symbiotic bacterium.

Signs seen in dogs with heartworm infection range from no signs (majority of dogs) to weight loss, reduced ability to exercise, cough, difficulty breathing, and formation of abdominal fluid. Heart murmurs and arrhythmias may occur. The severity of signs depends on the number of worms, length of infection, and the dog’s reaction to the heartworms. Heartworm numbers have been reported to range from 1 to over 250. The worms can produce a toxic substance, induce an immune reaction, cause damage to the lining of the blood vessels, and in severe cases create blockage of blood vessels. Dead heartworms can block arteries and form emboli.

Diagnosis is made using blood to test for adult heartworm antigens. Dogs with a positive antigen test generally have a second confirming test performed followed by a test to look for the presence of microfilaria. Additional evaluation includes other blood tests and chest X-rays. This information is important for planning treatment. Depending on the severity of infection and clinical signs, complications associated with treatment can be challenging. Melarsomine has been approved for treatment of adult heartworms but it is in critically short supply. The American Heartworm Society has developed guidelines for treatment and care when melarsomine is unavailable.

It is clear that heartworm infection can cause serious problems and be challenging to treat. However, preventive steps can be taken, including monthly administration of a preventive product such as Heartgard, Interceptor, or Revolution.

Our region does not yet have an endemic heartworm problem even though we live in a river valley. However, common recommendations for reducing the individual risk here include annual testing for heartworms even when your dog never leaves the valley, use of the monthly preventive year round or at least during mosquito season, and not traveling with your dog to a heartworm area during mosquito season. Annual testing can be important to make sure that infections are identified and treated early especially because a small number of dogs become positive even while taking the preventive.

Alternatives to the monthly preventive drugs have been proposed including the use of herbs like walnut and wormwood, and homeopathic remedies. Unfortunately, none of these proposed approaches have been adequately evaluated to determine their efficacy. In addition, there is concern about toxicity with the use of wormwood-containing products. Therefore, it is important to be fully informed before using these products.

Heartworm infection is a serious problem that is preventable. Even though we live in an area that is relatively free of heartworm problems, it is still valuable to discuss this issue with your veterinarian and establish a proactive approach for protecting your dog companion.

— Ron Carsten, DVM, Ph.D, CVA was one of the first veterinarians in Colorado to use the integrative approach, has lectured widely to veterinarians, and has been a pioneer in the therapeutic use of food concentrates to manage clinical problems. In addition to his Doctor of Veterinary Medicine, he holds a Ph.D. in cell and molecular biology and is a Certified Veterinary Acupuncturist. He practices integrative veterinary medicine in Glenwood Springs.